Crystallization of Co-based amorphous alloys under the impact of the ion irradiation and recovery of the amorphous phase

Crystallization behavior of the amorphous phase in the Co–Fe–B–Si alloy system was investigated by X-ray diffraction technique under the influence both of thermal and irradiation activation. Crystallization starting temperature decreases by 200–300 ^°C under the influence of the irradiation. After partial crystallization of the amorphous alloy under the action of the irradiation annealed below the thermal crystallization temperature, the amorphous phase was again formed while annealing the alloy without irradiation. During irradiation and thermal annealing formation of the new metastable phase has been discovered with the diffraction pattern similar to the previously discovered cubic aperiodical phase in the Fe–Nb–Cu–B–Si alloy.     

Effect of Ni-addition on the crystallization behavior and the oxidation resistance of Zr-based metallic glasses below the crystallization temperature

The crystallization behavior of Zr_65.0Al_7.5Ni_10.0Cu_17.5 metallic glasses by addition of Ni with 753 K annealing treatment and its effect on the oxidation resistance around the supercooled liquid region at 663 K were studied. By annealing at 753 K, the nanocrystalline phase of bct-Zr₂Cu precipitates was observed in the Zr_65.0Al_7.5Cu_27.5 specimen, while microstructures consisting of finer nanocrystalline bct-Zr₂Cu, fcc-Zr₂Ni and Zr₆Al₂Ni formed in the Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen. The oxidation resistance of the melt-spun Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen was improved by addition of Ni, which is evidenced by less mass gain and thin oxide scale. The microstructural refinement by the formation of numerous nanocrystalline phases of bct-Zr₂Cu, fcc-Zr₂Ni and Zr₆Al₂Ni from the matrix resulted in an improvement of the oxidation resistance, whereas a relative coarse nanocrystalline phase consisting of bct-Zr₂Cu exhibited fast oxidation along grain boundaries. Although the oxide species for both specimens were composed of a large amount of CuO/Cu₂O, some tetragonal and monoclinic-ZrO₂ as well as a minor amount of the oxide state of Cu³⁺, the amount of oxides especially for ZrO₂ in the Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen was lower, which was probably due to suppressed oxygen diffusion in ZrO₂.     

Atomic critical potentials and structure of non-single-well potentials in glasses

The microscopic nature and structure of atomic local non-single-well potentials in amorphous materials are considered. It is shown that the nature of such potentials is related to the local potential instability in an appropriate critical mode of the atomic motion. The occurrence of critical potential with unusually small spring constants is predicted. Some simple microscopic models are discussed in detail. It is shown that the non-single-well potentials practically reduce to critical double-well potentials in real amorphous materials. Two different types of the double-well potentials in glasses are revealed. Some predictions are given for the distribution densities of the parameters describing tunneling states in the critical double-well potentials and for some effects in amorphous materials.     

The thermal stability and activation energy of crystallization of (Cu61.8Zr38.2)1−xAlx bulk metallic glasses

The present paper reports on the thermal stability and activation energy of crystallization of bulk metallic glasses (BMGs) (Cu_61.8Zr_38.2)_1−xAl_x. The (Cu_61.8Zr_38.2)_1−xAl_x composition series, prepared by copper mould suction casting into bars with a diameter of 3 mm, form BMGs with an e/a range of 1.24–1.3. These BMGs manifest increased thermal stability with increased e/a ratios. The activation energies (ΔE) of crystallization as derived from thermal analysis at different heating rates follow a similar tendency to that of the thermal stability, indicating stronger short-range ordering with increasing e/a ratios. The optimum BMG Cu_58.1Zr_35.9Al₆ exhibits the highest thermal stability and the largest ΔE.     

Influence of superheat before quench on the structure and stability of NiP metallic glasses studied by neutron scattering techniques

The temperature dependent structure of Ni₈₁P₁₉ metallic glasses, rapidly quenched from melts which have undergone different heat treatments, has been investigated by small angle neutron scattering and neutron diffraction. All samples contain in the as-quenched state particles or clusters of a very wide range of sizes. These are amorphous regions of different compositions in a large composition range and/or crystallites of another Ni composition than the matrix, such as Ni₃P, Ni₅P₂ or Ni₁₂P₅, or even “voids”. The results can be interpreted within the concept of a heterogeneous structure existing in eutectic melts in a certain temperature range. Comparison is made with results from the literature on samples from the same ribbons investigated by calorimetry as well as by fractography and Weibull analysis of mechanical properties.     

Effect of X-ray irradiation on solarization and crystallization of photosensitive glasses containing Ce, Sb, Sn and Ag

The effect of X-ray irradiation on the solarization and crystallization of a lithium silicate glass was investigated in this research. The results showed that the X-ray power has a significant effect on the amount of solarization, and the crystallization temperature of the glass. Applying an X-ray power of 2400 W on glass containing the photosensitive elements of Ce, Sb, Sn and Ag led to reduction of the DTA crystallization peak temperature from 704 to 590 °C. UV spectroscopy showed that solarization of irradiated glasses containing Ce, Sb and Sn ions was responsible for formation of Ag clusters and reduction of crystallization temperature of glasses. Microstructural analysis also showed that the solarized glasses had a finer microstructure relative to that of the non-irradiated one. These differences attributed to changes in crystallization mechanism observed in the non-irradiated glass.     

Surface structure and properties of NiZr model metallic glasses: A molecular dynamics simulation

A systematic study of the surface structure and properties of NiZr model metallic glasses is reported using atomistic simulations. It is found that at low temperatures below the glass transition temperatures, the surface retains the amorphous structure and the surface energy γ is significantly lower (∼50%) than that of the corresponding crystalline alloy constituents. The variation of alloy concentration has little effect on γ; but increase in cooling rate and annealing temperature can lead to large decrease in γ. At elevated temperatures, γ increases with temperature and surface melting occurs at a temperature about 30% below T_g. At all temperatures up to T_g, the surface remains atomically smooth.     

Multi-scale structuration of glasses: Observations of phase separation and nanoscale heterogeneities in glasses by Z-contrast scanning electron transmission microscopy

Chemical fluctuations are probed in glasses obtained with different thermal histories using scanning transmission electron microscopy in high angle annular dark field mode. Direct imaging of the glass structure is obtained at the sub-nanometer scale with Z-contrast. Macroscopic glass-in-glass separation is probed in a slowly quenched melt where chemical resolution allows the determination of the regions associated with Zr/Zn atoms. In a quickly cooled glass, exempt of macroscopic phase separation, a segregation of Zr/Zn atoms is still evidenced but on a different length scale, suggesting either the beginnings of glass separation or intrinsic features of the glass structure. Glass inhomogeneities must be taken into account to have a quantitative evaluation of nucleation processes. These “nano”-heterogeneities can be associated with the nucleation of zirconia phases, giving important clues to understand the nucleation pathways and the structural role of nucleating agents in aluminosilicate glasses.     

Partitioning of gadolinium and its induced phase separation in sodium-aluminoborosilicate glasses

Phase separation in sodium-aluminoborosilicate glasses was systematically studied as a function of Gd₂O₃ concentration with transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) methods. Gadolinium-induced phase separation in the glasses can be consistently explained by proposing that Gd cations partition to the borate-rich environments and subsequent agglomeration of the Gd-borate moieties, or short-range ordered structural groups, in the glass. Agglomeration of the Gd-borate rich environments is further discussed within the context of excess metal oxides, [Na₂O]_ex or [Al₂O₃]_ex=|Na₂O-Al₂O₃|, and excess B₂O₃, [B₂O₃]_ex, available for incorporating Gd cations. Results showed that agglomeration of the Gd-borate rich environments occurred at a much lower Gd₂O₃ concentration in the glass without [Na₂O]_ex or [Al₂O₃]_ex and at a significantly higher Gd₂O₃ concentration in the glass with either [Na₂O]_ex or [Al₂O₃]_ex. Assuming 1BO₄:1Gd:2BO₃ (based on literature-reported Gd-metaborate structure) as a local Gd-borate environment in glass, we introduced the saturation index of boron, SI_[B]=Gd₂O₃/(1/3[B₂O₃]_ex), to examine the glass susceptibility to Gd-induced phase separation for all three alkali-aluminoborosilicate systems. While our results have provided some insight to the glass structure, they also provide insight to the mechanism by which the metal oxide is dissolved into the melt. This appears to occur predominately through boron complexation of the metal oxide.     

Influence of composition in the crystallization process of Fe75−xNb10B15+x metallic glasses

The devitrification process of Fe_75−xNb₁₀B_15+x (x = 0, 5, 10) metallic glasses produced by melt-spinning has been analyzed by calorimetric, microstructural and magnetic measurements. The experimental results show large differences in the behavior of these alloys as a function of composition. The alloy x = 0 undergoes a primary crystallization process separated in two stages while only one is observed for alloys with x = 5 and x = 10. This difference, observed by DSC, is correlated with a microstructural change in the phases that precipitate. For alloys x = 5 and 10, Fe₂₃B₆ and bcc-Fe are identified after the first calorimetric peak. In the sample x = 0, bcc-Fe and an unidentified phase precipitate in the first peak but massive crystallization of bcc-Fe is observed after the second stage. Finally the thermal dependence of magnetization has been measured and the Curie temperatures for the metallic glasses are determined. The change of these quantities with heat treatment and composition is discussed.     

The effects of microalloying on thermal stability,mechanical property and corrosion resistance of Mg-based bulk metallic glasses

(Mg₆₅Cu₂₀Y₁₀Zn₅)₉₈ M₂ (M = Ti, Cr) bulk metallic glasses with diameter of 3 mm were prepared by copper mold casting. The effects of Ti or Cr on thermal stability, mechanical property and corrosion resistance were systematically investigated. It's shown that the Glass forming ability of the Mg₆₅Cu₂₀Y₁₀Zn₅ bulk metallic glasses slightly decreases with the addition of the elements. Whereas, the strength and corrosion resistance significantly increase. The superior corrosion resistance of the amorphous sample containing Ti or Cr is presented for the forming of homogeneous passive layer with highly protective oxides.     

Evolution of Yb environment and luminescence properties under ionizing irradiation in aluminoborosilicate glasses

Yb-doped aluminoborosilicate glasses were irradiated with 2.3 MeV electron and gamma rays at different doses ranging between 10⁴ and 2.6 × 10⁹ Gy and the local structure around the Yb³⁺ ions has been studied using a combination of NMR, EPR and photoluminescence spectroscopic techniques. The spectroscopic results indicate the presence of two distinct Yb³⁺ sites in these glasses and their relative fractions depend on both the Yb concentration and the irradiation dose. These two sites can be attributed to Yb³⁺ ions with and without Yb next-nearest neighbors. The evolution of the shape of the ⁷F_5/2 → ⁷F_7/2 infrared emission band under irradiation is explained by a preferential reduction of the Yb³⁺ ions with Yb next-nearest neighbors at high integrated doses (> 10⁸ Gy). This interpretation is supported by a strong decrease of the visible cooperative luminescence under irradiation due to the reduction into the Yb pairs. In other terms, it shows an interesting effect of ionizing irradiation on the Yb clusters. A low intensity emission band appears in irradiated samples at 390 nm that can be attributed to Yb²⁺. Finally, the decrease of the Yb³⁺ fluorescence lifetime observed under irradiation appears to be primarily due to electronic interaction of these ions with Non-Bridging Oxygen Hole Centre defects created by the ionizing radiation.     

Structural changes of amorphous Pd80Si20 with neutron irradiation,with an interpretation in terms of the imperfections in the structure of amorphous solids

Wide and small-angle X-ray scattering measurements were made, as well as differential scanning calorimetry, for amorphous Pd₈₀Si₂₀ before and after irradiation by neutrons (5 × 10²⁰ neutrons/cm², > 1 Mev), and the structural changes thereby produced are described.Before irradiation the structure is well assimilated with a dense random packing (DRP) structure except that the coherence between neighboring Pd atoms extends farther than expected from the DRP structure, the coherent length estimated from the X-ray scattering being about 8 Å.With irradiation the coherent regions of 8 Å diameter get separated from each other by a mean distance of 20 Å, thus producing structural inhomogeneities which do not exist at all before irradiation. The glass transition temperature is found to be increased by about 10 K after irradiation.The effects of irradiation are interpreted as being due to the structural imperfections produced by particle bombardment. The DPR structure is adopted as a model structure for the perfect amorphous solids.     

Electronic properties and atomic structure of (Ti, Zr, Hf)---(Ni, Cu) metallic glasses

The available experimental and theoretical data on the electronic density of states at the Fermi level, n(E_F), of non-magnetic melt-quenched TE_100-xTL_x amorphous alloys (Te = Ti, Zr or Hf; TL = Ni or Cu) and for the corresponding crystalline counterparts is reviewed. First, the low-temperature specific heat and superconductive data are summarized, in order to derive experimental n(E_F) values. A comparison with n(E_F) data from theoretical band structure calculations shows good overall agreement, both qualitative and quantitative. By taking into account recent results on the different structural modifications of Ti, Zr and Hf metals, an extrapolation of the composition dependence of n(E_F) in TE-TL glasses to pure amorphous TE metals suggests that the local structure of these TE-TL amorphous alloys may be described by an fcc-like atomic arrangement. Some other reported results, which supply further evidence for the proposed fcc-like local structure in this type of metallic glass, are also discussed.     

Influence of the B and P content on the thermal stability and crystallization of cast iron based bulk amorphous alloys

The possibility of synthesizing bulk amorphous alloys with additions of B and P to commercial cast iron (Ci) of the chemical composition Fe_81.5Si_3.8C₁₄Tm_0.7 (at.%) was investigated. The effect of the B and P concentration on thermal stability, bulk glass forming ability (BGFA) and microstructure was studied by calorimetric (DSC and DTA) measurements as well as by X-ray diffraction. With the addition of 8.42 and 12.17 B to the commercial Ci bulk amorphous alloys with very wide supercooled liquid regions (77 and 81 K, respectively) could be produced. The partial replacement of Ci by B and P atoms increases the thermal stability and the BGFA. The kinetics of crystallization was investigated both by linear heating and by isothermal DSC measurements. The changes in the microstructure of the crystalline phases formed during linear heating were studied during further isothermal annealing at 833 K.     

Aligned crystallization in glasses: An analysis of heat transport and interface kinetics

An analysis of heat flow effects which are significant in the development of aligned microstructures in thermally crystallizable glasses is presented. Microstructures developed by heat treatment of a representative glass composition in the CdGeAs system have been interpreted on the basis of a heat flow analysis model developed in this work. The utility and limitations of the current analysis in problems related to interfacial kinetics and microstructure evolution in directionally crystallized glasses are discussed.     

Part 1: Kinetics and mechanism of the crystallization process

The kinetics of spontaneous crystallization of sodium chloride from aqueous‐ethanol solutions were studied. During the crystallization the electrical conductance and optical transmission of the supersaturated solutions were measured automatically. For monitoring of the total surface of growing potassium chloride crystals at the crystallization the turbidimetric method was used. The growth rate and activation energy were determined. The crystal growth rate was proportional to supersaturation. When the volume fraction of ethanol in solution increased from 14.85 to 29.72%, the activation energy of the growth process did not change and was about 50 kJ· mol^‐1. Aggregation of the crystals was found. The aggregation kinetics of the crystals may be described approximately by the famous Smoluchowski equation for coagulation of colloidal particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation and stability of amorphous molecular systems: As the prototype of functional amorphous organic materials

Structural changes in vapor-deposited amorphous states of simple organic compounds were studied by Raman scattering, X-ray diffraction, and light interference in the sample. Two types of processes, namely direct crystallization and structural relaxation leading to glass transition, were observed depending on the flexibility of the molecule. Gradual relaxations were also found to occur in amorphous states of butyronitrile and 1,2-dichloroethane at temperatures much lower than their T_g or crystallization temperature. The observed structural changes are discussed by referring to the high enthalpy of the vapor-deposited amorphous systems.     

Thermal stability, spectra and laser properties of Yb: lead-zinc-telluride oxide glasses

A series of new glasses of 70TeO₂-(20 − x) ZnO-xPbO − 5La₂O₃-2.5K₂O-2.5Na₂O (mol%) doped with Yb³⁺ is presented. Thermal stability, spectra and laser properties of Yb³⁺ ions have been measured. It found that 70TeO₂-15PbO-5ZnO-5La₂O₃-2.5K₂O-2.5Na₂O composition glass had fine stability ((T_x−T_g)>190 °C), high-stimulated emission cross-section of 1.25 pm² for the ²F_5/2 → ²F_7/2 transition and existed measured fluorescence lifetime of 0.94 ms and the broad fluorescence effective linewidth of 72 nm. Evaluated from the good potential laser parameters, this system glass is excellent for short pulse generation in diode pumped lasers, short pulse generation tunable lasers, high-peak power and high-average power lasers.     

Torsion and tension stress induced transformation of surface magnetic structure in Co-rich amorphous microwires

The effect of torsion and tensile stress on the surface magnetic properties in Co-rich glass covered amorphous microwires has been studied. The magnetization reversal in microwires has been studied by magneto-optical Kerr effect. Calculations of hysteresis loop was performed taking into account the existence of helical magnetic anisotropy in the outer shell of the microwire. The comparative analysis of the results of the experiment and the calculation demonstrates that the observed transformation of the surface hysteresis is related to the variation of the angle of the helical surface structure.